Catalytic conversion of molecular nitrogen to ammonia is a critical component of the global nitrogen cycle. This chemical transformation is carried out via a combination of natural biological processes and anthropogenic activities. Biological generation of ammonia via nitrogen reduction occurs primarily by natural processes mediated by organisms resulting in nitrogen fixation necessary for sustaining life. Industrial processes for nitrogen reduction, on the other hand, are believed to generate ammonia on a roughly equivalent scale to the biological sources. For example, the annual industrial production of NH3 via nitrogen reduction is estimated to be approximately 200 million tonnes per year and supports a range of commercial applications including fertilizers, decontamination and/or sterilization agents, and precursors for the production of other nitrogen containing chemicals.
Industrially, the majority of ammonia is produced in commercial quantities via the Haber-Bosch process. This process involves the reaction of nitrogen gas with hydrogen gas in the presence of a solid-state catalyst to produce ammonia. A range of catalysts have been developed for the Haber-Bosh process including iron-based catalysts and ruthenium-based catalysts. The Haber-Bosch process is resource intensive as it typically involves high pressures (˜100 atm), high temperatures (˜450° C.), as well as an industrial supply of precursor hydrogen gas. Given these requirements, ammonia production via industrial processes represents a significant amount of total energy consumed throughout the world each year (estimated to be as much as 1%).
In contrast, the transformation of nitrogen gas to ammonia by biological processes occurs efficiently under ambient conditions. This process is mediated by organisms, such as diazotrophs, and involves cofactors of nitrogenase enzymes rich in Fe and S and which may additionally feature Mo and V. Substantial research has been directed toward understand the chemical mechanism of the iron-molybdenum cofactor, for example, that provides for catalytic conversion of nitrogen into ammonia at low temperatures and pressures. Despite extensive research over the last several decades, the exact mechanism for biological reduction of nitrogen to ammonia remains uncertain.
The development and study of functionally biomimetic catalysts provide useful tools for understanding the mechanism involved in nitrogen reduction by nitrogenase enzymes. Such functionally biomimetic catalysts also provide a potential pathway for development of industrially relevant molecular catalysts providing for the reduction of N2 to NH3. An example of this has been approach is the development and characterization of transition metal molecular complexes capable of binding and functionalizing N2 under ambient conditions.
Molecular systems for reduction of N2 to NH3 have traditionally focused on Mo centers given the presence of Mo in the most thoroughly studied iron-molybdenum cofactor. Tri-amido amine Mo and phosphine-pincer Mo complexes, for example, have been demonstrated to provide moderate catalytic efficiencies for reduction of N2 to NH3 at ambient temperature and pressures. Attention has also been more recently directed to a potential role of iron as an active N2 binding site in iron-molybdenum cofactor given the understanding that iron is the only transition metal essential to all nitrogenases. This approach is also supported by recent spectroscopic and biological data implicating iron, as opposed to molybdenum, as the active site of N2 binding in the iron-molybdenum cofactor.
It will be appreciated from the foregoing that there is currently a need in the art for improved molecular catalysts and, methods capable of the facile conversion of nitrogen to ammonia. Specifically, molecular catalysts are needed for nitrogen reduction providing useful catalytic efficiencies and turnover under conditions less stringent than those in the conventional Haber-Bosh process.